Injection molding machines for plastics and rubbers or visco-elastic materials generally are known, in which what is essentially a single screw extruder has a reservoir formed in its cylindrical barrel in front of the screw and in which the screw, which is movable axially as well as rotatably, serves to f ill this reservoir and, upon being pushed backwards to a preset point, then serves as a piston in a positionally triggered, fast forward movement to inject the contents of the reservoir into the mold. This was first patented as DBP 858 310 in the 1960's.
A non-return valve at the tip of the screw is introduced in some constructions to enhance its injection action, though at the expense of some additional dead space.
This simplest plasticizing unit featuring basically a straight-through path along the axis of the screw, has as a drawback:
That the axial motion of the screw may not exceed about 4xD (D=screw diameter) in order not to have too short a length of the barrel in which the screw performs what plasticizing action it is capable of. In other words, the shot-volume is severely limited, size for size of the screw and barrel diameter.
To ameliorate this difficulty, two developments are known:
A. The complete extruder fills up a reservoir situated in front of its outlet opening, within a cylindrical housing in which the complete plasticizing unit (cylinder+screw+inlet) can be moved as a piston. PA0 B. The extruder and reservoir are on separate center-lines and thus entirely separate but for a valved connection, where these center-lines may intersect, at the outlet of the extruder and leading to the injection reservoir which is provided with a piston for the actual injection.
Both of these developments, while usable for larger shot-volumes, result in much greater mechanical, driving and control-difficulties, and, especially the cylinder/ram version is said to present difficulties in setting the desired shot-volume as exactly as is possible with the simple screw-ram device.
Moreover, all three types generally have plain transport screws and provide the plasticizing action mainly by heat transfer from the barrel. As the practice of injection-molding has become more sophisticated, the demands on the uniformity of the plasticized material have increased, so that at this time non-uniformities in the different batches of what is ostensibly the same compound have shown negative effects in the molded products.
Accordingly, there is now a demand for a mixing-homogenizing action in the plasticizing units.
Furthermore, if two chemically different compounds could be introduced into the extruder and continuously mixed therein so completely as to form a very fast, and possibly lower temperature-reacting compound, the cycle times in the molds could be reduced materially. This would yield considerable improvements in productivity.
It is an object of this invention, for the simplest plasticizing unit of an injection molding machine where the ram action is carried out by the screw itself:
to eliminate the need for a non-return valve on the screw tip, and thereby the dead volume caused by the valve;
to maintain the same effective plasticizing length of the screw irrespective of the preset shot volume, --i.e. to overcome the limitation on the mold filling stroke, s&lt;4xD;
to provide improved accuracy of shot-volume control both for large and particularly also for small shot-volumes;
to provide a mixing-homogenizing action together with more uniform plasticization which is adjustable to fit different requirements; to provide a mixing action adequate for the blending of two components which, on account of their chemistry, together provide a much faster vulcanization possibly also at lower temperature, and thus molding cycle time, than is possible for a single component compound on account of the need for storage and handling.
In order to achieve these objects, this invention provides:
A plasticizing screw and barrel comprising an inlet, a compression zone and a plasticizing zone embodying a Transfermix geometry according to British Patents No. 842,692 or No. 888,864 or No. 1,585,531 or No. 1,585,532 with a cylindrical interface between the lands of the grooves in the screw and in the barrel and having the generic feature that in the screw the helical groove varies in cross-section from full area to zero area and in the barrel the cross-section of groove varies from zero area to full area, whereby in the said plasticizing zone the material is transferred from the screw into the said barrel while being mixed and plasticized;
and a short second zone in which the flow area in the barrel outside of the cylindrical envelope of the screw reduces from full area to zero area for the material to be returned to within the cylindrical interface of the said barrel, the said barrel continuing with the same interior cylindrical shape to form a reservoir for plasticized material with a shaped end comprising a channel, generally centrally situated, for connection to the injection-mold, the said screw being axially moveable for the injection-action and with its section of zero area (depth) of its helical groove, on this having traversed said second zone, fitting the cylinder exactly like a piston for the said injection-action, trigger-means being provided to set off the said injection action and to stop rotation of the screw when the reservoir space is full.
It is a feature of a Transfermix plasticizing zone that its action may be intensified by throttling the flow of material through it, --the throttle in this case being formed by the said second (return) section between its ending, where the grooves in the barrel have zero depth, and the position on the screw where its grooves also have zero depth, where through the axial position of the screw for its plasticizing action the cylindrical area through which the material passes from the barrel grooves into the barrel may be adjusted to anything between a full opening--equal to the full cross-sectional area of the grooves in the barrel--and practically zero, this setting of the axial position of the screw being effected by positioning-means preferably integrated into the means providing for the axial motion of the injection-stroke.
In order to provide for the feature of adjustability of the shot-volume which in most cases will be required in practice, a preferred embodiment of the above may have the said screw comprise a central, generally cylindrical, core which is axially moveable relative to it, whereby with the screw in its axial position for plasticizing, the position of the said core is adjustable to define a shot-volume between a minimum of the annular space with the core fully extended through the cylindrical reservoir space in the barrel and touching the outlet end, and a maximum with the core fully retreated, equal to the full cylindrical reservoir space, the said core being adapted to actuate a trigger-means for setting off the injection-action of the screw as soon as the material, having filled the preset shot-volume, starts to push the core from its preset position backwards into the screw.
The said core and the said screw are provided with joining means which act to move the core forward with the screw as one body as soon as the head of the screw in its forward injection motion reaches the head of the core. The head of the screw and that of the core may be formed to present one continuous surface which may be shaped to exactly fit the exit end of the barrel-reservoir, in order to allow substantially no dead space with the screw forward. Alternatively the said trigger-means may be a pressure sensitive device fitted in the reservoir space and reacting directly to an increased pressure exerted by the material after having filling the preset shot-volume, in order to eliminate any motion of the core from its preset position prior to injection commencing and thereby to provide extreme accuracy in the shot-volume.
In another preferred embodiment, the head of the core may be left adjacent to the outlet end of the barrel, thereby forming a valve preventing a back-flow.
Moreover, in that position it may be adapted to exert a positive pressure on the material in the mold and to maintain such pressure for a period. In such an embodiment, after completion of the shot, the screw only will be withdrawn into its plasticizing position and will then start rotating for its next plasticizing action, filling up the annular space between the barrel and the core. After a timed delay, the core will be moved back to assume its preset position for the next shot. On account of the proven superior plasticizing efficiency of the Transfermix geometry, the screw lengths for plasticization need not be increased over the conventional, but may rather be reduced, enabling existing presses to be adapted by fitting the above units.
With the Transfermix being capable of operating also at higher screw speeds than usual in conventional plasticizing screws without causing excessive temperatures, relatively short screw lengths become possible, so that bigger diameter screws with bigger cores can be used also for smaller shot-volumes and cycle times, insofar as they depend on the time required to fill the reservoir, e.g. in carousel arrangements, can be reduced accordingly.